Three-phase reactor modeling with significant backmixing in the liquid phase using a modified cell model (MCM)

One parameter models have been extensively used for three-phase reactor simulation when backmixing is present in the liquid phase. Axial dispersion and cell models are among the most commonly employed, using the dispersion coefficient and the number of cells, respectively, as parameters that account for the liquid backmixing. Because of the mathematical complexity of the system usually at hand Some symplifying assumptions are made to obtain a tractable problem. Some of them involve the thermodynamics or physical property estimation. This work introduces a modified cell model (MCM) for three-phase reactors with significant backmixing in the liquid phase. Different phenomena are accounted for in the model: interfacial mass transfer limitations, adiabatic operation, phase distributions. All these effects are evaluated sequentially within a repetitive cell unit, following the criterion that numerical complexity is proportional to the number of equations and to the degree of coupling among them. In the case of MCM, the number of cells determines the degree of coupling among the effects and it is not related to the extent of backmixing as in classical models. Liquid phase mixing is introduced by considering a macroscopic recycle in the corresponding phase between the first and last cell. The numerical solution of this model is accomplished through an adequate computer program and the rise of a commercial process simulator that provides an important link to thermodynamics and transport properties estimation packages. This is a new technique that allows the user to build a complex conceptual model with little development effort. The present work is focused in bubble column slurry reactors. In most cases the chemical reaction occurs between agas reactant dissolved in the liquid phase and a liquid reactant in the surface of a solid catalyst suspended in the slurry. One of such applications is the hydrocracking of heavy oil fractions. Its adiabatic operation is modeled as a function of the global gas-liquid mass transfer coefficient far the reacting gaseous species; particle diameter, inlet solids concentration, superficial gas velocity and recycle ratio that accounts for liquid backmixing. (C) 1998 Elsevier Science Ltd. All rights reserved.